Wouldn't people like to buy a book about the lives of Haerta, Ganonron and Jephton?

[The Pilgrim]

If they weren't the focus of the story... maybe they could be valid villains (although not Big Bads), and I'd be glad to see them cross swords with the Scribblers. After all, the only interesting thing is that, despite being epic level baddies who zombified people / conquered nations quite a lot... they eventually got killed and defeated (Haerta looked fairly young when she died, too). So it wouldn't be stories about omnipotent evil beings that never get what they deserve.

The sorcerer is still silly.

In addition, all of the three appearing, and at once, shouldn't work; it's somewhat implied that they never meet while alive, and should they have, chances are good that they should have went straight to each other's neck.

One of them as an individual villain, as a minor boss for a short story ark (such as Bozzok, the Golem-maker, the young Black Dragon, the Bandits, etc...) would work. The sorcerer, as a one-shot joke would work fine.